AVDT: Use L2cap to enforce security

Instead of using btm_sec_mx_access_request() to enforce security, let
L2cap enforce security for us through btm_sec_l2cap_access_req().

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Test: A2dp audio streaming
Change-Id: I23143f9030df39a61e4896c0c777274078da5c83